Ultrasound (US) and US contrast agents (UCA) can be used in sonoporation to deliver therapeutic compounds noninvasively into specific target cells. The applicability of sonoporation is hindered by the lack of knowledge regarding the mechanism. Inertial cavitation (1C) of UCAs is the assumed mechanism; however, most of the data provided in the literature are circumstantial evidence and do not definitively indicate that 1C is the cause of the sonoporation. UCAs can also produce microstreaming, shear stresses, and liquid jets as a result of linear and nonlinear oscillations at pressure levels well below the threshold for 1C. Because these physical phenomena could also cause biological effects, a rigorous study to determine the biophysical mechanism of sonoporation must be conducted. Therefore, the long-term objective of this project is to elucidate the relationship between the UCA and sonoporation. The specific aims of this proposal are to determine the role, if any, 1C plays in eliciting sonoporation. Specifically, this study will directly compare the results of sonoporation to the independently determined collapse threshold of the UCA. A series of approaches will be used to elucidate the biophysical mechanism; each approach will vary one of the state variables and a series of factorial-based studies will vary 2 or 3 variables. At ambient (1 atm) and overpressure (1.5 to 5 atm) conditions, the UCA collapse thresholds and the sonoporation activities will be measured. At overpressure the UCA dimensions will decrease and thus the UCA collapse thresholds will likely change. Other variables will also be used including US frequency and UCA (OptisonTM and Definity[unreadable]1/2). These studies will directly compare the results of sonoporation to the collapse threshold of the UCA for a range of conditions and evaluate if inertial cavitation is necessary for sonoporation. The fundamental clinical goal of gene therapy and drug delivery is to develop approaches that deliver therapeutic material to the appropriate cells in the patient in a way that is specific, efficient, and safe. Sonoporation, unlike other methods of transfection or chemotherapy, combines the capability of enhancing gene and drug transfer with the possibility of restricting this effect to the desired area and the desired time. Thus, sonoporation is a promising drug delivery and gene therapy technique, limited only by lack of understanding regarding the biophysical mechanism that results in the cell membrane permeability change; which is what this project seeks to elucidate. [unreadable] [unreadable] [unreadable]